Abstract The American Cancer Society estimates that in 2016 there will be 1.7 million new cancer cases diagnosed and 600,000 cancer deaths in the United States. Although advances have been made in the treatment of cancer with surgery, radiation, chemotherapy, and with biologics, there is a critical need to develop novel treatment approaches with promise for improved selectivity, potency, and efficacy. This project introduces a new and previously untested platform strategy to enhance the efficiency of delivery of macromolecules to the cytoplasm of targeted cells. The central component of the strategy is the use of ?catch-and-release? monoclonal antibodies (CAR), with pH-dependent receptor binding. The CAR antibodies are conjugated to highly potent macromolecular toxins (CAR-toxin) and to endosome escape peptides (CAR-EEP). Combined administration of CAR-toxin and CAR-EEP conjugates allows efficient, targeted delivery and release of the toxin in the cytoplasm, enabling highly selective and potent anti-cancer efficacy. To pursue this project, we have developed a novel catch-and-release anti-carcinoembryonic antigen (CEA) antibody (10H6), with high affinity CEA binding at physiological pH, and with dramatically reduced binding at acidic pH. Preliminary data have been generated to assess the binding, cytotoxicity, and pharmacokinetics of 10H6, which in sum, strongly support the feasibility of the work. An interdisciplinary team has been assembled to evaluate this approach, with three experimental aims that will: (1) examine and optimize the utility of catch and release conjugates for cytoplasmic delivery of macromolecules to cancer cells in vitro, (2) test hypotheses regarding the tumor selectivity of in vivo disposition of the antibody conjugates, and (3) evaluate the safety and efficacy of the catch-and-release mAb conjugates in the treatment of mice bearing human xenograft tumors. The project, if successful, will establish a new targeting concept, with potential utility in enhancing the efficiency of cytoplasmic delivery of virtually any macromolecule to any type of cancer.